Thermocouples, devices that convert heat energy into electrical energy, are well known. They are most commonly used for measuring temperatures where conventional thermometers or other temperature measuring instruments cannot be used.
A thermocouple utilizes the thermoelectric effect. Essentially, a thermocouple is comprised of two dissimilar metals, usually in wire form, joined together at a location called the hot junction. The other ends (leads) of the metals are usually ultimately connected to a voltage sensing device, i.e. a volt meter or potentiometer; these leads are at a known reference temperature called the cold junction The temperature differential between the hot junction and the cold junction creates a small but measurable electrical potential between the dissimilar metals. By measuring this potential, the temperature sensed by the hot junction may be ascertained.
The conductors running from the hot junction of the thermocouple are often encased in a metal sheath. The sheath includes electrical insulation generally comprised of an electrically resistive metal oxide.
Mineral insulated metal sheathed (MIMS) thermocouple elements are normally made from separately produced, initially small diameter tubing of typically 25.4 mm (1 inch) outside diameter and cold drawn element wires of about 4.0 mm (0.16 inch) diameter. A number of involved procedural steps are first required to manufacture the narrow tubing and wires to these sizes. The tube may be made from INCONEL.RTM. alloys, NICROBELL.RTM. alloys, austenitic stainless steels, etc., and the negative and positive wire elements may be made from CHROMEL.RTM. alloys, ALUMEL.RTM. alloys, NISIL.RTM. alloys, nicrosil alloys, CONSTANTAN.RTM. alloys, etc. MIMS thermocouple cables are available, for example, as "K", "N", "T", "J" and "E" types.
The wire elements are then inserted into the tube. An insulator, usually magnesium oxide in the form of prefused crushable beads, is introduced into the tube in part to space the wire elements, to prevent them from contacting the tube, and provide some mechanical integrity to the thermocouple cable. The beads include holes to accommodate the wire elements The entire assembly is then swaged, rolled or cold drawn to the required final small diameter--usually on the order of about 3.0 mm to 6.0 mm (0.12-0.24 inches).
The aforementioned method is intrinsically expensive; it requires numerous preparatory steps; and is time consuming.
Representative prior art techniques, predominantly employing swaging, may be found in U.S. Pat. Nos. 2,703,335; 3,065,286; 3,121,038; 3,353,260; 3,463,674; and 4,778,537.